1. Field of the Invention
This invention relates to a process for removing pollutants from gas streams; more particularly, it relates to a process for removing sulfur oxides from waste gas streams containing same. In a preferred embodiment, this invention relates to an improved double alkali process for removing sulfur dioxide from gas streams containing same.
2. Description of the Prior Art
For various reasons, it is necessary and desirable to remove pollutants such as sulfur oxides, particularly sulfur dioxide (SO.sub.2), from gas streams containing same. A prime source of such gas streams comprises the waste gases derived from the combustion of sulfur-containing fuels, although they can also be generated by other gas producing processes.
Many different processes have been suggested for removing sulfur oxides such as sulfur dioxide from waste gases including, for example, scrubbing with a solution of sodium carbonate or sodium hydroxide; scrubbing with lime or a limestone slurry; or by the so-called double alkali process. The first process is simple but its application is limited since the resulting dissolved solids must be treated and fed to waste in a liquid stream. The lime or limestone scrubbing process produces a low-solubility solid waste, but although the cost of the scrubbing chemicals is low, the capital requirements are high due to the high circulation rates required and the possibility of deposition of solid scale in various parts of the system which necessitates periodic cleaning and/or replacement.
The double alkali process remedies some of these deficiencies and generally comprises scrubbing the sulfur oxide containing gas with an alkali such as a sodium or potassium solution, followed by the addition of another alkali such as lime to at least a portion of the resulting scrubber effluent to regenerate the alkali scrubbing solution. The primary advantage of the double alkali process is that the scrubbing step produces only soluble materials and that insoluble materials are formed only in areas outside the scrubber. However, such double alkali processes of the prior art were deficient in that the regeneration efficiency was not high and a predominantly sulfite filter cake was produced which is difficult to handle.
One such double alkali process is disclosed in U.S. Pat. No. 3,911,084. As disclosed therein, a circulating aqueous extracting/scrubbing solution of sodium sulfite and sodium bisulfite is provided having a specified pH of 6-7. Make-up for the extracting solution may be obtained using a soluble alkali sodium compound solution with a pH of 8-12. Upon contact with the sulfur dioxide-containing gas stream, SO.sub.2 is absorbed by the extracting/scrubbing solution and an extract is formed having a relatively lower pH and a relatively higher sodium bisulfite content. At least a portion of the extract is regenerated in a single step by reaction with a calcium compound such as lime to form a slurry containing dissolved sodium sulfite and solid calcium sulfite and having a pH of 8-10. After separating solids therefrom, the regenerated sodium sulfite solution is mixed with the circulating extracting/scrubbing solution to maintain its pH at 6-7.
As in the above patent, most sodium-based processes utilize caustic (i.e., NaOH) or soda ash (i.e., sodium carbonate, Na.sub.2 CO.sub.3) as make-up to the scrubbing solution. Such a scrubbing solution always contains sodium bisulfite (NaHSO.sub.3) and sodium sulfite (Na.sub.2 SO.sub.3) since the reactions shown in the following equations (1) and (2) occur spontaneously and reaction (3) represents the main SO.sub.2 absorption reaction: EQU Na.sub.2 CO.sub.3 +NaHSO.sub.3 .fwdarw.2Na.sub.2 SO.sub.3 +CO.sub.2 +H.sub.2 O (1) EQU NaOH+NaHSO.sub.3 .fwdarw.Na.sub.2 SO.sub.3 +H.sub.2 O (2) EQU SO.sub.2 +Na.sub.2 SO.sub.3 +H.sub.2 O.fwdarw.2NaHSO.sub.3 ( 3)
In addition, the scrubbing solution may also contain sodium sulfate (Na.sub.2 SO.sub.4) due to oxidation of sodium sulfite by any free oxygen in the gases and also because of the possible formation of sodium sulfate during regeneration which is recycled to the scrubbing step. Therefore, as a result of the scrubbing or absorption step, an aqueous solution is produced containing relatively less sodium sulfite, relatively more sodium bisulfite and some sodium sulfate.
In the regeneration step, the calcium compound added, e.g., Ca(OH).sub.2, reacts with the sodium bisulfite forming solid calcium sulfite (CaSO.sub.3) and regenerating sodium sulfite as shown in equation (4): EQU 2NaHSO.sub.3 +Ca(OH).sub.2 .fwdarw.CaSO.sub.3 .dwnarw.+Na.sub.2 SO.sub.3 +H.sub.2 O (4)
However, depending on the pH and other conditions of the regenerating reaction, one or both of solid calcium sulfate and dissolved sodium sulfate may be formed. Specifically, at a pH of less than 8 and higher than 10, a greater portion of solid calcium sulfate tends to form rather than solid calcium sulfite but since the former is more water soluble than the latter and hence tends to go back into solution as sodium sulfate, while more calcium sulfite precipitates, a solid phase equilibrium is reached. Hence, depending predominantly on the pH of the regeneration, some sodium sulfate may be formed in the thickener and recycled to the scrubbing step with the regenerated sodium sulfite. In fact, the sodium sulfate concentration is permitted to equilibrate in the process disclosed in the aforementioned U.S. Pat. No. 3,911,084, in order to reduce the undesired removal of sulfite/bisulfite values with the solid precipitate.
Other processes for removing sulfur dioxide from waste gases are disclosed in, for example, U.S. Pat. Nos. 3,775,532; 3,653,812; 3,477,815; 3,485,581; 3,542,511; 3,607,033; 3,622,443; and 1,271,899.